Electrophotographic developing powder cloud generating process and apparatus



Nov. 12, 1957 R. E. HAYFORD 2,812,883

ELECTROPHOTOGRAPHIC DEVELOPING POWDER CLOUD GENERATING PROCESS ANDAPPARATUS Filed Dec. 30, 1954 07w) for: RICHARD E. HAYFORD United StatesPatent ELECTROPHOTOGRAPHIC DEVELOPING POWDER CLOUD GENERATING PROC- ESSAND APPARATUS Richard E. Hayford, Pittsford, N. Y., asslgnor to TheHaloid Company, Rochester, N. Y., a corporation of New York ApplicationDecember 30, 1954, Serial No. 478,760 7 Claims. (Cl. 222-193) Thisinvention relates to an electrophotographic developing powder cloudgenerating process and apparatus. In electrophotography the exposedelectrophotographic image plate is developed by passing a flow ofsuitable developing powder over the exposed surface. The powder adheresto the surface inversely substantially in proportion to the degree ofexposure to light thus producing a positive image. The grain size of thedeveloped image depends, of

course, upon the particle size of the developing powder- As inphotography generally a fine grain image is desirable.Electrophotographic developing powder may be made with a very smallparticle size, such as in the order of an average diameter of 1 micron,and if this. particle size can be maintained during the necessaryhandling of the powder until the development is completed a fine grainimage results.

One way for developing the image is to discharge or eject the powderover the latent electrophotographic image by means of a moving stream orfiow of a gaseous carrier fluid, such as air, in which the powder isdispersed in the form of a cloud. This practice requires the generationof the cloud from the powder and this should be done in such a manner asto disperse the individual powder particles uniformly throughout themoving air flow. If the particles tend to agglomerate and form largesized particles, representing a number of the powder particles, the useof this powder cloud cannot produce a developed image that is as finegrain throughout as is potentially possible. Prior art powder cloudgenerating processes and equipment have sometimes tended to produce thisagglo-rnerating effect with its attendant effect on the developed imageand although it may be possible to deagglomerate the flowing particlesit obviously is more desirable to eliminate completely or materiallyretard agglomeration in the first place.

With the foregoing in mind, one of the objects of the present inventionis to provide a powder cloud generating process and equipment which ismore successful than usual in producing a powder cloud free fromagglomerations of individual particles and which represents more truly acomplete dispersion of the developing powder. Another object is toprovide a practical and inexpensive powder cloud generating process andequipment which produces a dispersion of the developing powder particlesin a substantially uniform pattern of individually free particles. Otherobjects may be inferred from the following.

Referring now to the accompanying drawings, which are mainly schematicin character:

Fig. 1 schematically showing in vertical cross section one example ofthe invention, and Fig. 2 being a similar kind of view but showing asecond example of the invention.

Having reference first to Fig. 1, certain elements shown are the same asone form of the prior art electrophotographic developing cloudgenerator. Thus, there is an enclosure 1 provided with an inlet 2 whichis supplied with a compressed carrier gas, for the cloud, under superatmospheric pressure such as in the order of 60 p. s. i. This carriergas customarily is compressed air and will be referred to hereinafter asair with the understanding that any compressible gaseous medium may beused which is of such a physical and chemical character as not tointerfere with the desired development and which can carry thedeveloping powder in the form of a cloud.

A suitable support 3 for the developing powder is located inside of theenclosure 1. Any prior art support of the travelling type is suitable,such as a travelling web of fabric impregnated with the developingpowder or, as is illustrated, a rotating disk which supports the powdereither by impregnation or otherwise. In Fig. 1 the disk 3 is connectedby a shaft 4 with a source of rotary motion 5 so that the disk 3 rotatescontinuously within the enclosure 1 and, of course, while bathed in thecompressed air or other carrier gas. As is known to those skilled in theelectrophotographic art, a travelling support, such as the disk 3, maybe kept continuously impregnated with electrophotographic developingpowder, such as powdered charcoal or other material capable of acceptingan electrical. charge, in various ways. Althoughnot illustrated, one wayis to maintain a pile of the powder on the support 3 at a localizedlocation where the powder is maintained by a scraper or the like so thatas the disk turns away from the powder a uniform amount of the latter isdistributed over the disk.

The prior art powder cloud generator includes the above elements. Inthis generator it is necessary that the travelling support 3 be perviousto the air or other carrier gas. The powder cloud is produced bylocating a powder cloud outlet directly on the support 3 so that thecompressed air, existing like a bath within the enclosure 1, drivesthrough the pervious support, picks up the powder particles and carriesthese particles through the outlet in cloud form for transmission to thedeveloping location. In one particular form of prior art device thecloud outlet is in the form of a plurality of turbulence tubes arrangedwith their inner ends pressing on the travelling powder support so thatthe powder is driven through the turbulence tubes. Such tubes actuallyare tubes having a sufficiently small diameter so that the fluid flowproduces turbulent conditions within the tube causing the powder cloudtravelling through the tubes to be tribo-electrically charged.

Now with the above prior art kind of generator and cloud creatingpractice there are various connected disadvantages. For example, thedeveloping powder, which may have an average particle size in theneighborhood of 1 micron in diameter, is driven into cloud formation bythe compressed air driving the lowermost particles against the uppermostparticles and so on up through the particles forming the mass conveyedby the support 3, the particle size and physical characteristics of thepowder tending to cause agglomeration of the particles thus driventogether so that the resulting powder cloud contains a certainpercentage of relatively large particles. Obviously the developed image,resulting from the use of this powder, is not as fine grain in characteras is possible potentially. Another disadvantage is that it is necessaryto use a pervious material for the travelling support 3, the smallparticle size of the powder more or less demanding that such a supportbe in the form of cloth or other fabric made from threads spun fromfibrous material. This, in turn, introduces the problem that lint may bedriven up through the turbulence tubes along with the powder cloud beinggenerated, which is naturally undesirable.

According to the present invention the powder cloud outlet conduit forthe enclosure 1, it being possible that this outlet may comprise eithera single tube or a plurality of tubes such as the turbulence tubes, hasits opening or openings located in the enclosure 1 so that this openingis adjacent but is spaced an appreciable distance from thesupport 3.With this arrangement there is established a zone of pressure, betweenthe powder cloud outlet opening or openings and the support 3, which isrelatively low in pressure as compared to the average pressure existingthroughout the enclosure 1 due to the introduction of the compressed airor other gas through the inlet 2. .With such an arrangement the powderparticles are literally sucked from the support 3. Since the powderparticles are bathed with the compressed air or other fluid and sincethis gaseous medium must exist in at least some small amount between theparticles, the reduction in pressure effected by the describedarrangement results in floating off, picking up, or otherwise carryingaway the uppermost particles first followed progressively by the lowerparticles on 'down through the layer of powder on the support 3 orimpregnated therein. With this new concept the support 3 need no longerbe pervious to the carrier gas, it being possible to use a solid metalsupport, for example, over which the powder is .spread in a relativelythin layer. Due to the particle size of the powder such a layer wouldordinarily be many particles deep in any event. The same result isachieved if the support is in the form of the prior are pervious clothtype, for example.

With the above in mind, reference to Fig. 1 shows that the powder cloudoutlet is illustrated as comprising a vertically. arranged, elongated,cylinder 6 having a bottom wall 7 in which a relatively large number oforifices or openings 8 are formed. The arrangement is somewhat like thatof the top of a saltcellar, for example. Thebottom wall 7 is spacedslightly above the support 3 so that the above described eifect isobtained. The turbulence tubes 9, required to tribe-electrically chargethe powder or desirable for other reasons, connects with the inside ofthe cylinder 6 by being connected through the top wall of this cylinder.

Now with this illustrated new generator the described action is secured.The electrophotographic developing powder carried on or impregnated inthe support 3 is moved continuously below but spaced from the orifices8. Compressed air or other gas is continuously supplied to the enclosure1 through the inlet 2. The turbulence tubes 9 lead to the developmentlocation and hence to atmospheric pressure, so there is a flow throughthe orifices 8 developing a localized zone of low pressure between theseorifices and the support 3. As the latter continuously moves a freshsupply or layer of the developing powder into this zone, the powderparticles are sucked or floated upwardly through the holes 8. The formertendency to agglomerate is eliminated or reduced.

The cylinder 6 functions as an expansion chamber for the cloud jetswhich enter this cylinder by way of the orifices 8. Thus there isconsiderable turbulence inside of the chamber 6 and a tendency, ofconsiderable practical effect, for the powder cloud to homogenize orbecome uniform without agglomeration of the powder particles. This is ofimportance because, among other reasons, those orifices which areclosest to the axis of the rotating disk 3 naturally receive lesspowder. In the case of a belt, it may be that the belt is not uniformlyimpregnated with the powder so, again, some of the orifices 8 willreceive more powder than others or, in other words, a powder" cloud ofgreater density. The expansion chamber 6 functions to consolidate thepowder cloud and produce a single powder cloud of substantially uniformdensity. i

It now becomes obvious that the turbulence tubes 9, regardless of theirrelative positions with respect to the disk 3, and regardless of unevenpowder impregnation or distribution, all receive or are charged with apowder Cal cloudwhich is in each tube of substantially the same densityas in all other tubes.

Due to the fact that the powder particles are picked up, lifted from, orsucked from the support 3, there is less tendency for lint to be carriedalong in the event this support is of the impregnated cloth type such asmight be used particularly if a belt type of powdersupport or conveyoris used.

It can beseen that in all instances the effect is that of moving a layerof the powder particles, while they are bathed in the carrier gas, intoa zone which is of lower relative fluid pressure, this causing thepowder particles to be sucked or lifted off by the expansion of the airor other .gas in ,which they are bathed. The prior art effect of oneparticle driving against another is avoided and, therefore, thegenerated powder cloud is less liable to represent agglomeratedparticles to any great extent such as might be suflicient to be visiblein the development image. 3 i

In the example shown by Fig. 2, the wall portion 7, shown by the exampleof Fig. 1, is now in the form of a large diameter annular wall 11forming a central hole 12 of-large diameter in the sense that it islarge as CO..& pared'to any of the orifices 8 of the Fig. 1 example. Asin the case of the first example this wall 11 is, of course, within theenclosure 1 and the opening 12 connects with a conduit which carries theresulting powder cloud to the location where development is desired.This Fig. 2 example serves to emphasize the fact that the powderparticles are continuously introduced to a zone of relatively lowpressure which causes a flow of the compressed air or other carrier gasso as to effect the described sucking or floating action. It isto beappreciated that assuming that the powder particles are of adequatelysmall size to permit a powder cloud of aerosol-like character, that withthisinventio'n such aerosol-like cloud becomes a realistic possibility.This invention eliminates the prior art tendency toward agglomerationand, of course, reduces the problem of lint, when this is present, andvarious other troubles.

Referring more specifically to Fig. 2, an inner conical wall 13 extendsupwardly from the inside of the annular wall 12 and terminates with anopening or orifice 14 formed in the apex of the wall 13, and an outerconical wall 15 extends upwardly from the outer periphery of the wall11, this outer wall having an opening 16 formed in its apex whichconnects with a conduit 17 which carries the powder cloud to wherever itmay be required for use. If turbulence tubes are used they may besubstituted for this single relatively large tube 17 which isillustrated.

In the case of this second example, the expansion chamber is formed bythe relatively large space between the walls 13 and 15, the actionotherwise being somewhat as previously described.

In the operation of both devices the powder cloud carrier gas is flowedover this support carrying the power and is flowed to and through theopening in the generator enclosure through which the powder cloud isdriven. The expansion chamber, provided by both examples, givesthepowder cloud an opportunity to become more uniform. In both examplesthe entrance to the expansion chamber, represented by the total crosssectional area of the holes 8 or the cross sectional area of the hole14,

as the case may be, is relatively small as compared to the volume of theexpansion chamber; the volume of this expansion chamber both laterallyand longitudinally, with respect to the inlet opening thereto wherebythe cloud is introduced, is in both instances many times greater thanthe dimensions or cross sectional area of this inlet.

In the handling of a powder cloud it is customary to experience asettling or drifting of the particles when an enlargement in the flowpath of the cloud is encountered. However, with the present invention,involving the sucking principle described, no trouble has beenexperienced with such settling or drifting of powder particles.Apparently as the carrier gas is sucked from above the powder layer soas to carry the powder particles therewith, the various particles areindividually separated or floated in such a manner as to remain, ineffect, airborne at all times, the efiect of the expansion chamber beinglargely to smooth out or make uniform the density of-the powder cloud.

It is to be appreciated that the inlet 2 might be opened to theatmosphere and a suction applied to either the turbulence tubes 9 or thetubes 17, whereby to effect the previously described action. However, inaddition to technical problems such a procedure does limit the availablepressure ditferential to atmospheric pressure. In the actual practice ofthe present invention it has been customary, for example, to introduceair through the inlet 2 of the enclosure 1 at a pressure in theneighborhood of 60 p. s. i., the exhausting ends of the turbulence tubes9 or the tubes 17 going to atmospheric pressure for all practicalpurposes.

I claim:

1. A powder particle dispersion process including forming a layer ofsuperimposed powder particles bathed in a gaseous fluid, forming a zoneof fluid pressure that is less than that of said gaseous fluid, andcausing said layer to travel so that one side thereof passes throughsaid zone and so that said gaseous fluid flows away from said layercarrying said particles therewith in a dispersed state for xerographicdevelopment purposes, the improvement comprising homogenizing themixture of particles in the gaseous fluid while maintaining theparticles in a substantially deagglomerated state prior to conveying theparticles for xerographic development purposes.

2. An electrophotographic developing powder cloud generator including anenclosure having an inlet for a compressed carrier gas for the cloud, asupport for the powder and which is located within said enclosure, anda. powder cloud outlet conduit for said enclosure and which has at leastone opening located in said enclosure adjacent to but spaced from saidsupport so as to establish a zone of pressure therebetween which is lessthan the average pressure throughout said enclosure when said inlet issupplied with compressed carrier gas, whereby to generate the powdercloud by sucking the powder from said support, said outlet conduitincluding an expansion chamber to homogenize the mixture of powder incarrier gas while maintaining the powder in a deagglomerated state.

3. An electrophotographic developing powder cloud generator including anenclosure having an inlet for a compressed carrier gas for the cloud, asupport for the powder and which is located within said enclosure, and apowder cloud outlet conduit for said enclosure and which has at leastone opening located in said enclosure adjacent to but spaced from saidsupport so as to establish a zone to pressure therebetween which is lessthan the average pressure throughout said enclosure when said inlet issupplied with compressed carrier gas, whereby to generate the powdercloud by sucking the powder from said support, said conduit including anexpansion chamber in which the powder cloud expands after leaving saidopening.

4. An electrophotographic developing powder cloud generator including anenclosure having an inlet for a compressed carrier gas for the cloud, asupport for the powder and which is located within said enclosure, anexpansion chamber having a wall portion located within said enclosureand adjacent to but spaced from said support and said portion having aplurality of orifices formed therethrough so as to establish a zone ofpressure between said orifices and said support which is less than theaverage pressure throughout said enclosure when said inlet is suppliedwith compressed carrier gas, whereby to generate the powder cloud bysucking the powder from said support, and conduit means connected tosaid chamber to conduct the powder cloud therefrom.

5. An electrophotographic developing powder cloud generator including anenclosure having an inlet for a compressed carrier gas for the cloud, asupport for the powder and which is located within said enclosure, anexpansion chamber having a wall portion located within said enclosureand adjacent to but spaced from said support, said wall portion beingannular and having inner and outer conical walls extending therefromaway from said support, said outer wall terminating with an outletpassage formed through its apex and said inner wall terminating with aninlet passage formed through its apex, said inner wall being shorteraxially than is said outer wall and the space therebetween defining saidexpansion chamber.

6. An electrophotographic developing powder cloud generator including asupport for said powder, conduit means for receiving said cloud andhaving, therefore, at least one receiving opening spaced adjacent tosaid support, and means for flowing a cloud carrier gas over saidsupport and to and through said opening, said conduit means including anexpansion chamber for the cloud.

7. In an electrophotographic developing powder cloud generator having anenclosure supplied with compressed gas and containing a travellingsupport charged with the powder and a plurality of turbulence tubesthrough which the generated cloud is forced, the combination therewithof a cloud expansion chamber interposed between said tubes and saidsupport and having at least one opening adjacent to said support andwhich is located within said enclosure so that the compressed gas flowstherethrough so as to create a pressure drop sucking the powder fromsaid support into said expansion chamber of driving on through saidchamber and into said tubes, said chamber having an expansion spacevolume many times greater than that of all of said tubes combined andalso many times greater than that of said opening.

References Cited in the file of this patent UNITED STATES PATENTS1,371,412 Davis Mar. 15, 1921 1,673,087 Morse June 12, 1928 2,360,464Arveson Oct. 17, 1944 2,533,331 Skinner Dec. 12, 1950 2,624,652 CarlsonJan. 6, 1953

